Conventionally, there has been utilized a gas sensor which detects the concentration of a specific gas contained in an object gas such as exhaust gas. For example, an NOX sensor, which detects nitrogen oxides (hereinafter referred to as “NOX”) as a specific gas, includes an oxygen concentration detection cell, a first oxygen pump cell, and a second oxygen pump cell, each composed of an oxygen-ion conductive solid electrolyte layer and porous electrodes formed thereon. This NOX sensor operates as follows. The first oxygen pump cell pumps oxygen out of a first measurement chamber such that the oxygen concentration detection cell outputs a constant voltage, to thereby control the oxygen concentration of the object gas within the first measurement chamber to a constant level. Upon application of a constant voltage between the electrodes of the second oxygen pump cell, the second oxygen pump cell pumps oxygen out of the gas introduced from the first measurement chamber into a second measurement chamber (the gas whose oxygen concentration has been adjusted by the first oxygen pump cell). The concentration of NOX within the object gas is detected on the basis of a current flowing through the second oxygen pump cell as a result of application of the constant voltage thereto (hereinafter, processing of detecting the concentration of NOX within the object gas will be referred to as “detection processing”).
In the case where the concentration of NOX contained in, for example, exhaust gas discharged from an internal combustion engine of an automobile is detected by use of an NOX sensor, the gas present in the second measurement chamber is becoming a lean state close to the atmosphere, in accordance with passage of time from the stoppage of the previous operation of the internal combustion engine to restart thereof. In view of such a phenomenon, some NOX sensors are configured to perform, at the time of startup of the internal combustion engine, preliminary control so as to temporarily and rapidly pump out oxygen present in the second measurement chamber and oxygen contained in a porous electrode facing the second measurement chamber, to thereby bring the interior of the second measurement chamber into a predetermined low-oxygen-concentration state. Thus, a time before stable measurement of the concentration of NOX contained in exhaust gas becomes possible is shortened. For example, the preliminary control is performed as follows. After a gas sensor element is started, a constant voltage higher than that in ordinary control is applied between the electrodes of the second oxygen pump cell so as to temporarily and rapidly pump out oxygen present in the second measurement chamber (see, for example, Patent Documents 1 and 2).    [Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2001-281211    [Patent Document 2] Japanese Patent Application Laid-Open (kokai) No. 2001-141696